1. Field of the Invention
This invention generally relates to a digital facsimile communication system for transmitting and receiving a binary image signal and in particular to a digital facsimile communication system which can represent tone differences in a reproduced copy.
2. Description of the Prior Art
The digital facsimile is widely used primarily because of its faster transmission rate over the analog facsimile. Although the digital facsimile has many advantages, it cannot be directly applied for an original having tone variations such as a photographic picture. Because, the digital facsimile is structured to work on a binary image signal consisting of only two possible states "high" and "low", a tone difference cannot be represented without special signal processing technique.
The currently most promising prior art technique for representing tone differences in the digital facsimile communication is the so-called dither technique. In accordance with this technique, the threshold level to be used as a reference in converting an analog image signal into a binary image signal is varied at random or regularly for a single or a predetermined number of picture elements when scanning along the main scanning direction, i.e., along the lengthwise direction of a single array image sensor, and at the same time the threshold level is similarly varied at random or regularly with respect to the auxiliary scanning direction which is perpendicular to the main scanning direction. The dither-processed image signal is excellent in image reproducibility; however, it tends to decrease the rate of transmission.
In a digital facsimile system, the run length of either the "white" or "black" picture elements along the scanning line in the main scanning direction is converted into an appropriate code, usually according to the modified Huffman coding, in order to carry out data compression, thereby eliminating redundancy in the information to be transmitted. However, if the binary image signal is dither-processed, the transition from "white" to "black" or its reverse would multiply and the transmission rate would be significantly lowered in certain cases. Thus, it is often the case with the prior art digital facsimile to bypass its data compressor to transmit the binary image signal without carrying out data compression. In this connection, the CCITT standards as to facsimile communication also set down the non-compression mode as well as the compression mode as a possible mode of digital facsimile transmission. It should however be noted that the use of the noncompression mode of transmission is not advantageous because, in the case of transmitting the information contained on a single DIN A4-sized document, several mega-bits of picture elements on the average must be transmitted and it would take about several hundred seconds if such information is transmitted at the rate of 4,800 bps.